fix: allow custom components via plugin options

[alvarosabu]

Describe issue

Passing custom resolvers for bloks inside via the plugin was overwriting the rest of default resolvers, causing errors.

Also, custom resolvers where not rendered

import CustomComponent from './components/CustomComponent.vue';
...
app.use(plugin({
  resolvers: {
    components: {
      'custom-component': () => h(CustomComponent)
    }
  }
}));

Solution

This PR adds a condition to add defaultResolvers when options.resolvers is passed to createRenderer